Process for coating non-fibrous organic sheet material



July 10, 1962 R. J. BOYER 3,043,713

PROCESS FOR COATING NON-FIBROUS ORGANIC SHEET MATERIAL Filed May 1:5, 19:58

, INVENTOR ROBERT JAY BOYER ATTORNEY United States Patent 3,043,713 PROCESS FQR COATING NON-FIBROUS ORGANIC SHEET MATERIAL Robert Jay Boyer, Clinton, Iowa, assignor to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware Filed May 13, 1958, Ser. No. 734,903 5 Claims. (Cl. 117-62) This invention relates to coating non-fibrous, organic sheet materials and has as its primary object, a continuous process for coating films, webs or sheets of regenerated cellulose with a moisture-proofing composition at relatively high speeds. Other objects willappear hereinafter.

The objects are accomplished by a continuous process comprising the steps, in sequence, of passing the sheet material through a bath containing a liquid coating composition; removing excess coating liquid from the surface of the sheet material; drying the coated sheet material at a temperature of at least 190 F. cooling thecoated sheet material to a temperature of not lower than 60 F. but below 100 F.; and humidifying the dried coated sheet material by exposure to a moist atmosphere, preferably an atmosphere at a temperature of l58185 F. having a relative humidity of 85-99%. The process as described has been found most useful when coating at speeds of at least 250 yards/minute.

The FIGURE is a side elevation of one form of apparatus suitable for carrying out the process of the invention.

In the embodiment shown in the figure, uncoated regenerated cellulose film 10, the so-called base film, is unwound from the roll 11 and enters the coating tower 12 through an opening 13. It will be noted that the film enters at the lower portion of the dryer section 20 of the tower although this is not essential to the present invention. Here the film receives preliminary heating by the circulating hot air.

The film then passes over roll 14 and enters the coating section 30 of the tower through the slot 15. The film passes into the dip tank 17; around roll 18; and upwardly between doctor knives 19. The dip tank 17 contains a moisture-proofing coating composition dissolved in a solvent. The coating composition, in general, comprises a cellulose derivative, a resin, a wax and a plasticizer. The cellulose derivative may be a cellulose ester or cellulose ether, but is preferably nitrocellulose and represents 30- 70% of the composition. The resin may be a gum or a resin, either natural or synthetic, such as ester gum, rosin, gum mastic, a copal resin, etc. and represents -60% of the coating composition. The wax is preferably a colorless, odorless, high melting paraffin wax (i.e., having a melting point above about 140 F.) but may also be ceresin or beeswax and represents 1-10% of the coating composition. As a plasticizer, 540% of one or more of the following compounds may be used: tricresyl phosphate, triphenyl phosphate, diamyl phthalate, dibutyl phthalate, dicyclohexyl phthalate or castor oil. While the particular solvent must be determined for the particular composition used, the following provide a basic group from which selection of an appropriate solvent can be made: alcohol, ether, toluene, ethyl acetate and butyl acetate. The coating liquid is maintained at a temperature of 100-125 F.

To prevent premature gelation of the coating on the surface of the film, the doctor knives 19 should be located close to the coating bath in dip tank 17. The gap between knife blades should be about 1 inches with the edges overlapping slightly. The overlap is adjusted to provide the desired coating thickness. As an alternative to doctor knives, doctor rolls may be used in a similar manner. After passing between the doctor knives 19 where excess coating is removed from the film surface and the coating 3,043,713 Patented July 10, 1962 remaining on the film is made smooth, the film passes through an adjustable slot 21 into the dryer section 20. Hot air heated by the heater 23 to a temperature of at least 190 F., but which may be as high as 350 F., is circulated through this section by means of a blower 22. The air serves to dry the coated film and to carry the solvent vapor through the opening 24. The solvent vapor may be recovered from the solvent-air mixture in a separate operation.

After being dried in the dryer section 20, the coated film passes into the cooling section 33 through the slot 34. In the cooling section the film is cooled to a temperature of not lower than 60 F., but below F. This range of temperature is critical to the operation of the present process and its importance will be discussed in greater detailsubsequently. Cooling is accomplished by passing cold water through the roll 26 by feeding the cold water through inlet 36 and removing the water through outlet 37. The film passes over the cooled roll 26 and then downwardly through the slot 27 into the conditioning or humidifying section 25 of the coating tower.

In the conditioning section moisture is added to the base film. The amount added serves to replace the amount lost by the base film during drying, and to bring the moisture content of the film up to the desired level. This is accomplished by circulating moist air (air having a relative humidity of at least 85%) at a temperature of 158- 185 F. into the section at 28 and out at 29. The conditioned coated film passes under roll 31, out through opening 35, and is collected on roll 32.

It may be desirable to apply the moisture-proofing coating composition to one side only of the sheet material. This is easily accomplished by merely raising the dip roll 18 so that only a portion of the roll is below the level of the coating composition in dip tank 17. In addition, a doctor roll would replace the doctor knife contacting the uncoated side of the sheet material.

The result of the process is a smooth, adherent, moisture-proof coating on the regenerated cellulose film. The coated film is very pliable and free of appearance defects. The pliability of the film is due to its high moisture content. For films having a thickness of up to 1 mil, pliability is obtained with moisture contents of at least 7.5%. For thicker films, i.e. about 1.2-1.5 mils, a minimum moisture content of 7.0% has been found to be satisfactory. Sheet materials coated according to the present invention are suitable for use as packaging for food stuffs, textiles, hardware, etc., as bottle cap seals, as tank linings, etc.

The critical features of the process which serve to distinguish it from prior art processes are:

(1) Cooling the sheet material coated with a composition containing at least 1% paraifin wax to a temperature of 60-100 F. prior to humidification.

(2) Humidifying the coated sheet material at a temperature of 158185 F. in a moist atmosphere, i.e., an atmosphere having a relative humidity of at least 85%.

The following theory is offered to explain the surprisingly effective results of the present invention but the theory should not be construed to limit the scope of the invention. In the operation of the process, the heated coated film emerging from the top of the drying section of the coating tower is cooled to a temperature within the above mentioned critical limits. When the cooled film enters the humidifying section of the tower, moisture immediately condenses on the coated surface or surfaces of the film. Condensation of moisture serves to release heat. The amount of heat released, it is believed, must be sufficient to raise the temperature of the coating above the softening temperature of the wax in the coating, i.e., above about F., in order to permit the penetration of r, K) moisture. It the film is cooled in the cooling section of the tower to a temperature below 60 F., then the amount of moisture condensed and the accompanying heat released does not raise the temperature of the coating above 140 F.; the wax layer, therefore, remains (as it is in the final product) substantially impermeable to moisture; and the resulting moisture content in the product is not as high as necessary to provide the desired pliable product. If the film is cooled to a temperature above 100 F., then the surface is too warm and not enough 'moisture condenses initially; and, hence, the moisture that 'penetratesthe wax layer is not as high as necessary to provide the desired pliable product.

Although cooling has been accomplished in the foregoing description by passing the film over a cooled roll, numberedzfi in the figure, through which cold water or cold brine is circulated, other means for cooling the film may be used. Cold air jets to pass cold air through the cooling. section 33 of the coating tower may be used with similar results.

In the cfollowing examples the importance of the present invention will :be apparent, particularly for a process operated at coating speeds above 250 yards/ minute. In all the examples the procedure depicted in the figure was used with a 300 gauge regenerated cellulose film (approximately 0.9 mil thick) and the following moistureproofing composition in dip tank 17:

Parts By Percent in Compound Weight Coating Composition Nitrocellulose (11.4% nitrogen) 56. 2 48 Resin:

Petrex-7-HI" 1 11.7 15 Polypale WB z 5.8 Crystalline parafi'in (melting point 140 F.) 3.5 3 Plasticizer:

Dibutyl phthalate 19.9 34 Dicyclohexyl phthalate. 19. 9 Solvent:

Ethyl acetate". 573. 9 Toluene 285. Ethyl alcohol 24.1

1 A diethylene glycol ester of terpene-nialeic acid manufactured by Hercules Powder C0. in accordance with USP 2,236,546.

1 A sulfuric acid-modified wood rosin manufactured by Hercules Powder Co. in accordance with USP 2,017,866.

Example I The speed of the film through the coating tower was 330yards/minute. The temperature in the cooling section was about 85? F. The humidifying section was o 4 maintained between 165 F. and 175 F. and at a relative humidity of about The moisture content of the resulting coated regenerated cellulose film averaged 7.8% and the coated film was smooth and clear.

Example III The speed of the film through the coating tower was 420 yards/minute. The temperature in the cooling section was about 68 F. The humidifying section was maintained between F. and F. and at a relative humidity of about 90%.

The moisture content of the resulting coated regenerated cellulose film averaged above 7.5% and the coated film was smooth and clear.

As many widely dififerent embodiments can be made without departing from the scope of this invention, it is understood that the invention is not limited except as defined in the appended claims.

Having fully described the invention, what is claimed is:

1. A continuous process for coating regenerated cellulose film which comprises the steps, in sequence, of passing a web of regenerated cellulose film through a moisture-proofing composition containing 30-70% nitrocellulose and 1-10% paraifin wax dissolved in a solvent, said wax having a melting point of at least about 140 F.; driving off the solvent from the coated web at a temperature of at least 190 F.; cooling the coated web to a temperature of 60 F.-l00 F and, thereafter, humidifying said dried coated web to a moisture content of at least 7.5% by exposing said web to a moist atmosphere at a temperature of 158 F.- F.

2. A process as in claim 1 wherein the moist atmosphere during the humidifying step has a relative humidity of at least 85%.

3. A process as in claim 1 wherein the moist atmosphere during the hurnidifying step has a relative humidity of 85-99%.

4. A process 'as in claim 1 wherein the web is treated at a speed of at least 250 yards/minute.

5. A continuous process for coating regenerated cellulose film which comprises the steps, in sequence, of passing a web of regenerated cellulose film through a moisture proofing composition containing 30-70% of a cellulose derivative and .*110% of paraffin wax dissolved in a solvent, said wax having a melting point of at least about 140 F.; driving off the solvent from the coated web at a temperature of at least F cooling the coated web to a temperature of 60 F.-l00 F.; and, thereafter, exposing said web to a moistv atmosphere at a temperature of 158 F.185 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,826,699 Charch et 'al Oct. 6, 1931 2,159,152 Hershberger May 23, 1939 2,350,292 Sermattei et a1 May 30, 1944 2,354,060 Rhodes et a1. July 18, 1944 2,808,340 Learn Oct. 1, 1957 

1. A CONTINUOUS PROCESS FOR COATING REGENERATED CELLULOSE FILM WHICH COMPRISES THE STEPS, IN SEQUENCE, OF PASSING A WEB OF REGENERATED CELLULOSE FILM THROUGH A MOISTURE-PROOFING COMPOSITION CONTAINING 30-70% NITROCELLULOSE AND 1-10% PARAFFIN WAX DISSOLVED IN A SOLVENT, SAID WAX HAVING A MELTING POINT OF AT LEAST ABOUT 140*F., DRIVING OFF THE SOLVENT FROM THE COATED WEB AT A TEMPERATURE OF AT LEAST 190*F.; COOLING THE COATED WEB TO A TEMPERATURE OF 60*F.-100*F.; AND, THEREAFTER, HUMIDIFYING SAID DRIED COATED WEB TO A MOISTURE CONTENT OF AT LEAST 7.5% BY EXPOSING SAID WEB TO A MOIST ATMOSPHERE AT A TEMPERATURE OF 158*F.-185*F. 